Finding Faults

In 1811, the area around New Madrid, Mo., was rocked by one of the strongest earthquakes ever in America. Some geologists think the region, home to 15 million, is overdue for another. Replay of 1811 quake called likely

September 18, 2005|By David Kohn | David Kohn,SUN STAFF

ON DEC. 4, 1811, an enormous cataclysm shook the central United States. It happened again six weeks later, on Jan. 23, and again Feb. 7. Most scientists say that each of the three had magnitudes approaching or above 8.0, stronger than the 1906 San Francisco earthquake - stronger, in fact, than any California earthquake in recorded history. Vibrations from the quakes toppled chimneys hundreds of miles away, cracked sidewalks in Cleveland, rang church bells in Boston and caused the Mississippi to run backward.

The epicenter of the earthquakes was the western frontier of the country, what is now the Missouri Bootheel. At the time, the largest settlement in the region was New Madrid, a town of 250 inhabitants on the western bank of the Mississippi. The quakes demolished the town, pitching much of it into the river.

"The scene was terrible beyond description," a river man named John Bradbury wrote. "Our boat appeared as if alternately lifted out of the water, and again suffered to fall. The banks above, below and around us were falling every moment into the river. All nature seemed running into chaos."

Could such a disaster happen again? If it did, the damage would be exponentially greater. Back then, a few thousand people lived in the region; today the population is 15 million.

The prospect seems far-fetched, at least to most non-geologists, and as a result planning and funding have lagged, just as they had in New Orleans. But many researchers argue that another large earthquake is likely.

"If a larger earthquake does happen, it would have tremendous consequences," says Eugene "Buddy" Schweig, a geologist at the University of Memphis Center for Earthquake Research and Information (CERI). "And it's going to happen eventually."

Unprepared

Schweig and others warn that the region is unprepared, that its buildings, highways and schools are not built to withstand strong shaking. In a scientific paper published this summer, Tish Tuttle, another New Madrid expert, recommended that stricter earthquake provisions be added to the building codes of Memphis and other central U.S. cities.

The region seems an unlikely place for earthquakes. Most quakes occur in places where tectonic plates - the moving puzzle pieces that make up the Earth's surface - are grinding against each other.

The San Andreas Fault in California, where the North American and Pacific plates are at loggerheads, is a textbook example. New Madrid, however, lies in the middle of a plate, thousands of miles from the treacherous edges.

Earthquake-prone areas also tend to be hilly, a result of buckling caused by the ruptures. The New Madrid Zone, by contrast, is pancake flat. Its fissures are buried by a thick layer of Mississippi River sediment - several thousand feet deep in some places, so no one is even sure where the fault is.

But it is definitely there. About once every two weeks, the fault readjusts itself slightly; most of these tremors are too minute for humans to feel.

In 1843 and 1895, the fault also produced serious ruptures. Although neither quake was as large as the 1811-1812 monsters, both were powerful.

The latter measured 6.8, the same size as the 1995 Kobe, Japan, quake, which killed 5,000 people.

10 or 20 years away?

Like many researchers, Arch Johnston would not be surprised if a large quake occurred in the next 10 or 20 years.

The director of CERI, Johnston has concluded that the odds of a mid-6 quake occurring in the next 15 years are 63 percent; the odds of such a quake occurring within 50 years are nine in 10.

"The model predicts you should have a magnitude 6 here every 70 to 100 years," said Johnston, a tall, almost gaunt man who resembles a Civil War general. "We've seen zero since 1895. So that would suggest we're slightly overdue."

Unlike the San Andreas, which regularly cracks all the way to the surface, the New Madrid Fault, buried by sediment, almost never shows itself. (As far as anyone can tell, it zigzags from Marked Tree, Ark., to Cairo, Ill., a distance of 120 miles.)

As a result, those who study New Madrid cannot rely on direct observation. One way around this is to focus on the record of past earthquakes.

Remains of explosions

Tish Tuttle specializes in this field, which is known as paleoseismology. By her own estimate, she has floated more than a thousand miles of rivers, creeks and irrigation ditches around the Southeast and Midwest.

She has a thick head of tousled graying hair, and gestures constantly, trying to illustrate how rocks collide, erode, rise and disperse. She lives in Maine and works on contract for the United States Geological Survey, spending three or four months a year in the New Madrid Zone.